Documentation: add HID-BPF docs

Gives a primer on HID-BPF.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/hid/index.rst

@@ -11,6 +11,7 @@ Human Interface Devices (HID)
    hidraw
    hid-sensor
    hid-transport
+   hid-bpf
 
    uhid
 

MAINTAINERS

@@ -9097,6 +9097,7 @@ M:	Benjamin Tissoires <benjamin.tissoires@redhat.com>
 L:	linux-input@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/hid/hid.git
+F:	Documentation/hid/
 F:	drivers/hid/
 F:	include/linux/hid*
 F:	include/uapi/linux/hid*